Patent Literature 1 describes a filter device 2 for preventing an influence of dust, soot, and the like on a projector 3 provided in a poor environment, as shown in FIG. 1 thereof. The filter device 2 includes an external casing that includes an air intake window 10 covered by a filter unit 14, a fan 15 that blows cooling air onto the projector 3, and a discharge window 11 provided in front of a discharge port 6 of the projector 3. The projector 3 is set in the external casing 13, and these are attached to a lid plate portion connected to a ceiling hanger (both of which are not numbered) (paragraphs [0013], [0017], [0023], etc. in specification).